ACES Output Transforms VWG
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Meeting #64, August 3rd, 1pm PT
- Alex Fry: Not much to discuss. A couple of extra survey responses. No meeting next week due to SIGGRAPH. There’s been some tone scale between Pekka, Jed and Daniele. I a version of the ZCAM DRT with Daniele’s curve, where you can switch tone scales. The default is very close the the current MM curve for SDR.
- Pekka Riikonen: I just wanted to make the HDR and SDR match the MM tone scale. So I bolted Jed’s model onto Daniele’s curve to calculate the parameters. Now exposure matches the MMTC. The discussion is what exposure hits peak luminance. I went for 10 stops for all. Jed has varying values. Daniele hits peak at infinity.
- Nick Shaw: Jed picked values he liked and fitted a curve.
- Pekka Riikonen: I did the same for 10 stops. I prefer a constant exposure at peak, e.g. ACEScct 1.0.
- Alex Fry: So same dynamic range for SDR and HDR? Is that to get the same scene values from an inverse?
- Nick Shaw: Because ACES has historically let you first grade SDR then switch to an HDR OT, if you have the same DR and grade SDR to hit peak white, that becomes 4000 nits when you switch to HDR. My gut says those extreme HDR values should only come from very high scene values which you wouldn’t push SDR to in a grade.
- Pekka Riikonen: In that case Jed’s model would be better.
- Alex Fry: People want to hit white in SDR.
- Pekka Riikonen: Or we call it infinity and they land where they land.
- Nick Shaw: Some broadcasters mandate graphic white at 100% SDR. That would invert to huge scene values, with the associated issues where they act like lights when you scale them. Jed’s values are just what he felt reasonable, and he fitted a curve. We could start from a function, and decide we add x stops from the scene per stop of peak white. Daniele suggested something similar for mid grey – a tenth of a stop per stop of peak.
- Pekka Riikonen: My curve matches Jed’s curve except slightly in the shadows.
- Nick Shaw: That’s because Jed had his spring function spliced on at the bottom, and Daniele’s doesn’t?
- Pekka Riikonen: Yes. I have a function increasing exposure with peak lumniance. It closely matches the existing.
- Alex Fry: It’s good to match what we have. So we fix the issue but keep the look. Please keep reaching out for feedback. Lars, do you have any comment on my ?
- Lars Borg: Ideally you have one flag indoors, and one outdoors and brightly lit, and you make sure they look reasonably the same. Even here you see the renderings are different colors.
- Alex Fry: I got the flag colors off the web.
- Lars Borg: An indoor flag might not be tone mapped much, but an outdoor hard lit one would be tone-mapped more.
Meeting #63, July 27th, 1pm PT